The inhibition of phosphodiesterase 9A (PDE9A) is one of the current concepts to find new access paths to the treatment of cognitive impairments due to CNS disorders like Alzheimer's Disease. With the present invention, new compounds are presented that follow this concept.
Phosphodiesterase 9A is one member of the wide family of phosphordiesterases. These kinds of enzymes modulate the levels of the cyclic nucleotides 5′-3′ cyclic adenosine monophosphate (cAMP) and 5′-3′ cyclic guanosine monophosphate (cGMP). These cyclic nucleotides (cAMP and cGMP) are important second messengers and therefore play a central role in cellular signal transduction cascades. Each of them reactivates inter alia, but not exclusively, protein kinases. The protein kinase activated by cAMP is called protein kinase A (PKA), and the protein kinase activated by cGMP is called protein kinase G (PKG). Activated PKA and PKG are able in turn to phosphorylate a number of cellular effector proteins (e.g. ion channels, G-protein-coupled receptors, structural proteins, transcription factors). It is possible in this way for the second messengers cAMP and cGMP to control a wide variety of physiological processes in a wide variety of organs. However, the cyclic nucleotides are also able to act directly on effector molecules. Thus, it is known, for example, that cGMP is able to act directly on ion channels and thus is able to influence the cellular ion concentration (review in: Wei et al., Prog. Neurobiol., 1998, 56, 37-64). The phosphodiesterases (PDE) are a control mechanism for controlling the activity of cAMP and cGMP and thus in turn for these physiological processes. PDEs hydrolyse the cyclic monophosphates to the inactive monophosphates AMP and GMP. Currently, 11 PDE families have been defined on the basis of the sequence homology of the corresponding genes. Individual PDE genes within a family are differentiated by letters (e.g. PDE1A and PDE1B). If different splice variants within a gene also occur, this is then indicated by an additional numbering after the letters (e.g. PDE1A1).
Human PDE9A was cloned and sequenced in 1998. The amino acid identity with other PDEs does not exceed 34% (PDE8A) and is never less than 28% (PDE5A). With a Michaelis-Menten constant (Km) of 170 nM, PDE9A has high affinity for cGMP. In addition, PDE9A is selective for cGMP (Km for cAMP=230 [mu]M). PDE9A has no cGMP binding domain, suggesting that the enzyme activity is not regulated by cGMP. It was shown in a Western blot analysis that PDE9A is expressed in humans inter alia in testes, brain, small intestine, skeletal muscle, heart, lung, thymus and spleen. The highest expression was found in the brain, small intestine, kidney, prostate, colon, and spleen (Fisher et al., J. Biol. Chem., 1998, 273 (25), 15559-15564; Wang et al., Gene, 2003, 314, 15-27). The gene for human PDE9A is located on chromosome 21q22.3 and comprises 21 exons. To date, 4 alternative splice variants of PDE9A have been identified (Guipponi et al., Hum. Genet., 1998, 103, 386-392). Classical PDE inhibitors do not inhibit human PDE9A. Thus, IBMX, dipyridamole, SKF94120, rolipram and vinpocetine show no inhibition on the isolated enzyme in concentrations of up to 100 [mu]M. An IC50 of 35 [mu]M has been demonstrated for zaprinast (Fisher et al., J. Biol. Chem., 1998, 273 (25), 15559-15564).
Murine PDE9A was cloned and sequenced in 1998 by Soderling et al. (J. Biol. Chem., 1998, 273 (19), 15553-15558). This has, like the human form, high affinity for cGMP with a Km of 70 nM. Particularly high expression was found in the mouse kidney, brain, lung and liver. Murine PDE9A is not inhibited by IBMX in concentrations below 200 [mu]M either; the IC50 for zaprinast is 29 [mu]M (Soderling et al., J. Biol. Chem., 1998, 273 (19), 15553-15558). It has been found that PDE9A is strongly expressed in some regions of the rat brain. These include olfactory bulb, hippocampus, cortex, basal ganglia and basal forebrain (Andreeva et al., J. Neurosci., 2001, 21 (22), 9068-9076). The hippocampus, cortex and basal forebrain in particular play an important role in learning and memory processes. As already mentioned above, PDE9A is distinguished by having particularly high affinity for cGMP. PDE9A is therefore active even at low physiological concentrations, in contrast to PDE2A (Km=10 [mu]M; Martins et al., J. Biol. Chem., 1982, 257, 1973-1979), PDE5A (Km=4 [mu]M; Francis et al., J. Biol. Chem., 1980, 255, 620-626), PDE6A (Km=17 [mu]M; Gillespie and Beavo, J. Biol. Chem., 1988, 263 (17), 8133-8141) and PDE11A (Km=0.52 [mu]M; Fawcett et al., Proc. Nat. Acad. Sci., 2000, 97 (7), 3702-3707). In contrast to PDE2A (Murashima et al., Biochemistry, 1990, 29, 5285-5292), the catalytic activity of PDE9A is not increased by cGMP because it has no GAF domain (cGMP-binding domain via which the PDE activity is allosterically increased) (Beavo et al., Current Opinion in Cell Biology, 2000, 12, 174-179). PDE9A inhibitors may therefore lead to an increase in the baseline cGMP concentration.
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